The present invention relates generally to sulfonated polymer compositions that are suitable for producing polymer electrolyte membranes, electrodes and membrane electrode assemblies for use in fuel cells, in electrolysis cells, in dialysis equipment and in ultrafiltration and methods of synthesizing polymer compositions. More specifically, the present invention relates to innovative multi-block sulfonated poly(phenylene) copolymers, methods of making the same, and their use as a proton exchange membrane (PEM) in hydrogen fuel cells, direct methanol fuel cell, and in electrode casting solutions and electrodes.
Polymer electrolyte fuel cells (PEFCs) have great potential as an environmentally friendly energy source. Fuel cells are electrochemical energy converters which feature in particular a high level of efficiency. Among the various types of fuel cells, PEFCs feature high power density and a low weight to power ratio. The PEFC uses as its electrolyte a polymer membrane.
Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Fuel cells are attractive electrical power sources, due to their higher energy efficiency and environmental compatibility compared to the internal combustion engine. The most well-known fuel cells are those using a gaseous fuel (such as hydrogen) with a gaseous oxidant (usually pure oxygen or atmospheric oxygen), and those fuel cells using direct feed organic fuels such as methanol.
The polymer electrolyte membrane or proton exchange membrane (PEM) is an important aspect of any PEFC. PEMs are excellent conductors of hydrogen ions. The most widely used materials to date consist of a fluorocarbon polymer backbone, similar to Teflon®, to which are attached sulfonic acid groups. The acid molecules are fixed to the polymer and cannot “leak” out, but the protons associated with these acid groups are free to migrate through the membrane in the presence of water. With the solid polymer electrolyte, electrolyte loss is not an issue with regard to stack life. The potential power generated by a fuel cell stack depends on the number and size of the individual fuel cells that comprise the stack, and the surface area of the PEM.
In many fuel cells, the anode and/or cathode comprise a layer of electrically conductive, catalytically active particles (usually in a polymeric binder). A polymer electrolyte membrane is sandwiched between an anode and cathode, and the three components are sealed together to produce a single membrane electrode assembly (MEA). The anode and cathode are prepared by applying a small amount of a catalyst, for example, platinum (Pt) or ruthenium-platinum (Ru/Pt), in a polymeric binder to a surface that will be in contact with the PEM. Preparation of catalyst electrodes has traditionally been achieved by preparing an ink consisting of an electrocatalyst (either Pt or Ru/Pt) and Nafion® polymer (5% wt. solution dispersed in lower alcohol). The ink is applied to porous carbon paper using a painting technique, or directly depositing the ink upon the membrane surface, or pressing it onto the membrane like a decal.
A MEA of a hydrogen fuel cell typically accepts hydrogen from a fuel gas stream that is consumed at the anode, yielding electrons to the anode and producing hydrogen ions (protons), which enter the electrolyte. The polymer electrolyte membrane allows only the hydrogen ions to pass through it to the cathode while the electrons must travel along an external circuit to the cathode thereby creating an electrical current. At the cathode, oxygen is reduced by the electrons from the cathode and reacts with the hydrogen ions from the electrolyte to produce water. The water does not dissolve in the electrolyte and is, instead, rejected from the back of the cathode into the oxidant gas stream.
For the last 30 years the industry standard for the PEM component of a hydrogen or methanol fuel cell has been membranes based on fluorine-containing polymers, for example, the Nafion® material marketed by DuPont. Nafion® is a perfluorinated sulfonic acid polymer having a well-known structure. Nafion® is often used as a membrane material for fuel cells that operate at temperatures close to ambient. Further, Nafion® polymer membranes are hydrated and they have a proton conductivity of about 10−2 S/cm or higher.
The Nafion® membranes display adequate proton conductivity, chemical resistance, and mechanical strength. Some of the membrane's disadvantages are low hydrated glass transition temperature high methanol permeability in direct methanol fuel cells, humidity dependence on proton conductivity, and cost.
The low hydrated glass transition temperature of Nafion® membranes may cause them to creep in a working fuel cell when hydrated above 80° C. Creep may perturb contact between the membranes with the electrode or gas diffusion layers, and may also lead to pin hole defects. Additionally, there is a need to reduce the costs associated with such membranes.
Another limitation of Nafion® membranes occurs in applications in methanol fuel cells as Nafion® membranes are permeable to methanol. Methanol crossover is inversely proportional to membrane thickness. Direct transport of the fuel (i.e. methanol) across the membrane to the cathode results in losses in efficiency. Increasing the membrane thickness results in decreased methanol crossover. However, thicker membranes result in increased Ohmic losses and decreased fuel cell performance.
Membranes that lowered the rate of methanol crossover would allow the use of higher concentrations of methanol-water feed mixtures, which would increase catalyst efficiency, direct methanol fuel cell power output, and potentially fuel utilization.
In general, increasing the operation temperature of fuel cells is advantageous for several reasons. Higher operating temperatures in methanol fuel cells decrease the carbon monoxide poisoning of the electrocatalyst. Higher temperatures increase reaction kinetics of hydrogen oxidation on the anode and oxygen reduction on the cathode. However, as the temperature is increased, it becomes more difficult to keep the membrane hydrated. Dehydration of membranes is exacerbated by relatively thick membranes. Dehydrated membranes lose ionic conductivity and result in poor contact between fuel cell components due to shrinkage of the membrane. Therefore, improved performance of fuel cells could be achieved by reducing the thickness of the membranes, and improving the humidification state of solid PEMs, since water molecules can promote proton transport and thin membranes can reduce ionic resistance and Ohmic losses.
Additionally, the contact between the membrane and electrode affects the efficiency of a fuel cell. Interfacial resistance between the membrane and electrode causes Ohmic loss thereby decreasing fuel cell efficiency. Improving the membrane-electrode contact and continuity, wherein the membrane and electrode are cast from a composition having the same or similar polymer electrolytes, would improve the membrane-electrode interfacial resistance.
What is needed are compositions from which improved polymer electrolyte membranes, electrodes, and electrode casting solutions can be made that have improved performance at temperatures at about 80° C. and above, and preferably above 120° C. Operating at these temperatures would result in enhanced diffusion rates and reaction kinetics for methanol oxidation, oxygen reduction, and CO desorption thereby producing a more efficient fuel cell.
Novel sulfonated PEM membranes have been previously synthesized at Sandia National Laboratories (SNL) for the DOE fuel cell program, and later patented (U.S. Pat. No. 7,301,002, which is incorporated herein by reference). These previous PEM materials were composed of a sulfonated poly(phenylene) compound that was prepared by a Diels-Alder reaction; and which will hereafter be referred to by the acronym SDAPP.
Scheme 1 shows an example of the synthesis of a SDAPP polymer.

The polymerization reaction to make the unsulfonated parent polymer is an irreversible Diels-Alder reaction that is responsible for forming every other phenyl ring in the backbone. Due to the ambiguous regiochemistry of the reaction, a mixture of 1,4 and 1,3-substituted rings are formed. The parent polymer is then treated with a sulfonating agent (e.g., ClSO3H) to attach sulfonic acid groups on the para-positions of some of the pendant phenyl rings in Scheme 1. The number of sulfonic acid groups formed can vary from x=1-6, and thus the ion exchange capacity (IEC) can be controlled by varying the amount of sulfonating agent used.
The previous synthesis method, diagrammed in Scheme 1, is based on post-modification of a specific family of poly(phenylenes). However, there are several disadvantages to this method, including:                The position of acid groups on the backbone cannot be controlled;        Tailored morphologies, or block and graft architectures, cannot be generated; and        The chemical similarity of the aryl groups prevents spectroscopic determination of the segment length, which presents a barrier to nano-controlled morphology.        
What is needed, then, are improved synthesis techniques that enhance the microscopic self-assembly of differing hydrophobicity/hydropholicity phases in ion containing polymers. Control and self-assembly of hydrophilic domains in the polymer can lead to more efficient mobility of protons through the membrane, improving ionic conductivity.